1. Field of the Invention
This invention relates generally to the formation of a layer of crystalline silicon and, more particularly, relates to the formation of a layer of crystalline silicon having a (111) orientation on a (111) surface of lithium aluminum.
2. Prior Art
The formation of crystalline silicon by heteroepitaxial growth on intermetallic compounds in the sodium thallium family is described in U.S. Pat. No. 4,042,447 and in U.S. patent application Ser. No. 789,566, now U.S. Pat. No. 4,115,625. The sodium thallium type intermetallic compounds possess a dual diamond structure in which each element, e.g., lithium, when considered alone forms the diamond crystal structure. In effect two diamond crystal structures are interwoven with three dimensional congruency. Each of the diamond crystal structures is isomorphic with crystalline silicon. As taught in these patents, heteroepitaxial crystal growth is obtained by application of silicon atoms with sufficient energy to allow them to find a periodic bond chain position in a growing film of crystalline silicon. The growing film uses the diamond crystal structure of the heavier element as its pattern or template.
High quality films of crystalline lithium aluminum having a (111) orientation can be prepared as disclosed in co-pending application Ser. No. 872,625. It has now been determined that an optimum heteroepitaxial system within the heteroepitaxial systems described in U.S. Pat. No. 4,042,447 and in application Ser. No. 789,566, now U.S. Pat. No. 4,115,625, is the nucleation, growth and epitaxy of a film of (111) crystalline silicon on such a film of lithium aluminum. This subsystem constitutes the present invention. This specification teaches the reasons for the critical significance of this optimum subsystem. A further purpose of this invention is to provide the particular heteroepitaxial system of silicon on a non-silicon substrate which will lead to the development of cost effective photovoltaics.